There have been many different substances and methods developed in the past for treating wounds, depending upon the type and location and severity of the wound. A wound is generally defined as an injury to an area of the body of a human or animal. Although injury to the surface of the skin is the most well known type of wound, the surfaces of internal organs may also be wounded, such as during surgery, rupture of the spleen or liver, or resulting from traumatic blows to the body surface in the vicinity of an internal organ.
Medical practice characterizes wounds as chronic or acute, according to the persistence and severity of the wound. A chronic wound is one that is prolonged or lingering, rather than promptly healed. An acute wound is one that occurs relatively quickly, and heals relatively quickly as well. Tissue wounds may have a wide spectrum of manifestations, as small as merely an abnormal microscopic tear or fissure in tissue (or a surface thereof), or as large as the abrasion or ablation of the skin covering a substantial portion of the body, such as in a burn victim. Acute wounds covering a large or movable surface are usually the most difficult to guard from infection, and to heal.
Blood and bodily fluids include various substances that affect wound healing. The blood is the primary medium for delivering healing agents to the wound site, and for transporting foreign or harmful substances away from the wound. Whole blood is primarily comprised of three main types of cells suspended in a protein rich solution known as plasma. The three main cell types in whole blood are erythrocytes (a.k.a. red blood cells), leukocytes (a.k.a. white blood cells) and thrombocytes (a.k.a. platelets). The red blood cells are the iron-containing cells that facilitate the transport and transfer of oxygen to body tissue, and the removal of carbon dioxide. The white blood cells perform a variety of functions such as phagocytosis of foreign bodies and production of antibodies, and are primarily responsible for fighting infection and foreign substances within the blood or wound site. Platelets perform many functions such as plugging leaks in blood vessels and helping begin the process leading to the formation of a blood clot; platelets contain substances known as growth factors that facilitate the formation of new tissue.
Although there are several methods for separating whole blood into its various components, one of the most convenient and expeditious methods is accomplished by differentially centrifuging blood or some of its components (i.e., apheresis). Using apheresis, the red and white blood cells and plasma may be separated out and returned to the donor's or patient's body, leaving the sequestered platelets in essentially concentrated form for use in wound healing techniques. From blood extracted from a patient, the platelets may thus be obtained and activated for use on the same patient; methods of using a patient's own blood are called “autologous” or “autogenic” donor methods. Methods using blood donated by one or more third parties for use by a patient are called “homologous” or “heterologous” donor methods, or collectively called “allogenic” methods.
One of the proteins suspended in plasma is fibrinogen, which reacts with substances released into (or attracted by) wound sites to produce sticky strands of fibrin. Such reactions result in the cross linking of the fibrin strands to form a mesh that holds and supports the deposit or growth of other tissue materials at the wound site.
The wound healing process is generally considered to occur in several stages, generally known as the healing cascade. After tissue injury, platelets are among the first cells to appear in the vicinity of the wound. Activation of a platelet by an agonist such as thrombin, or other agonists such as those listed elsewhere herein, leads to the release of granule material from within the platelet. Such granulation activation results in the release of proteins known as growth factors, primarily concentrated in the alpha granules of platelets. These released growth factors stimulate the formation of new tissue; when applied to wounds, growth factors have been known to increase the rate of collagen laydown, vascular ingrowth, fibroblast proliferation and overall healing. The release of a protein known as platelet-derived growth factor (PDGF) is a chemotactic signal for monocytes, neutrophils and fibroblasts which then move into the wound, to begin the inflammatory stage of the healing process. During this time, monocytes secrete a number of factors including PDGF and transforming growth factor-β1 (also found in platelets), which recruits and activates fibroblasts, to begin the repair stage of the healing process. Subsequently, wound healing continues through the process of collagen remodeling within the wound.
Based upon the foregoing general scientific principles, already known in the field are wound sealants made from biological materials obtained primarily from tissue other than blood platelets. An example is wound sealants such as “fibrin glue,” which often are essentially a mixture of co-coagulants (thrombin and calcium), concentrated fibrinogen and other coagulation proteins. In most applications, the primary roles of fibrin glue are to seal wound surfaces to prevent loss of blood and other body fluids after surgery, and to provide adhesion between adjacent tissue surfaces. These products form a hard, cast-like covering over the area to be sealed, and tend to be non-yielding to limb movement.
The production of fibrin glue often requires obtaining fibrinogen from blood through a process known as cryoprecipitation, including both freeze-thaw cycles and relatively lengthy centrifugation of plasma in controlled environments, to concentrate the fibrinogen in sufficiently for use; the precipitant thus obtained is frozen to −20° to −30° centigrade before storage. These requirements make such materials unsuitable for application during the course of surgery, especially emergency surgery without an hour or more lead time. Moreover, to the extent this process depends upon the use of autologous biological materials, using this process shortly before or during surgery may result in the loss of crucial bodily fluids during a time when the patient's body is badly in need of such fluids. By contrast, substantially larger amounts of concentrated platelets can be more conveniently obtained within a matter of minutes from more recent methods of differential blood centrifugation not requiring freezing and without significant loss of bodily fluids.
While there has been much research concerning fibrin glue, this material belongs to a separate field from the present invention, primarily because fibrin glues typically contain cryoprecipitated proteins without platelets. The use of fibrin glue is discussed extensively in the scientific literature; for example, see the references cited in U.S. Pat. No. 5,585,007 issued to Antanavich et. al. on Dec. 17, 1996.
Wound treatment compositions derived from platelet enriched concentrates are known and possess certain advantages over materials without platelets such as fibrin glue. One reason is that natural wound healing agents are released by the platelets. Further, the concentration of platelets likewise allows for a concentrated amount of wound healing factors. Additionally, to the extent that the wound healing composition is made from the biological materials of the patient, the risks associated with heterologous donors (such as disease, immunologic reactions, or the like) are eliminated. Representative examples of platelet derived wound treatment compositions are described for instance in Hood U.S. Pat. No. 5,733,545; Knighton U.S. Pat. No. 5,165,938; and Gordiner U.S. Pat. No. 5,599,558.
Platelet concentrates are typically isolated by the process of differential centrifugation which essentially allows separating the patient's own blood into at least three different components: packed erythrocytes (red blood cells), plasma and platelet concentrate. Platelet concentrate can be combined with a solution of either sodium or calcium mixed with thrombin (“calcified thrombin”), which instantaneously form a composition of activated platelets that, when made with the necessary viscosity, can be utilized as a wound sealant. The chemical reactions and cascades that normally occur when thrombin is added to the concentrated platelets are indeed complex. See, for instance, Reeder, et. al., in Proceedings of the American Academy of Cardiovascular Perfusion, Vol. 14, January 1993. Such wound sealants typically set up into a hard mass covering the application site, thereby sealing the site against further blood loss and external contaminants.
There are a number of disadvantages associated with conventional wound compositions derived from platelet concentrates. For instance, activation of platelets leads to instantaneous hardening of the material and thus requires the physician to both activate and apply the platelet composition to the wound site within seconds of activation. Also, certain platelet compositions must be applied to the wound site on a daily basis and thus require regular blood withdrawal from the patient. The presence of a hardened mass at the wound site is undesirable because it impedes oxygen transport into the wound which is necessary for tissue repair. It may also create a favorable environment for the growth of pathogenic anerobic bacteria.
Accordingly, an improved platelet enriched wound treatment composition which avoids or diminishes the problems associated with typical platelet enriched wound compositions would be desirable.